An aerosol is defined as liquid or solid particles suspended in a gas and can be classified into smoke, dust, mist, fume and the like depending on the physical state of particles. The aerosol is detrimental to a human body and becomes a cause of pollution in various kinds of industries. For the purpose of accurate evaluation of the aerosol, collection and analysis of particles is becoming an important research subject.
Particles of the aerosol are measured by means of an optical particle counter (OPC) on a real time basis. Due to the scattering of a laser beam, the optical particle counter has no ability to accurately measure particles having a size of smaller than 60 nm. This means that the optical particle counter is not suitable for use in the fields requiring real time measurement of particles having a size of smaller than 60 nm, such as semiconductor production, medical chemistry, biology and genetic engineering.
A scanning mobility particle sizer (SMPS) is used in these fields to measure the aerosol on a real time basis. Particles of the aerosol are bipolar-charged by a neutralizer of the scanning mobility particle sizer and then supplied to a differential mobility analyzer (DMA). The differential mobility analyzer undergoes a change in voltage while the particles pass therethrough. The particles passing the differential mobility analyzer are affected by electric fields changing over time. Thus, those particles that have the same electrical mobility are extracted by the differential mobility analyzer.
The scanning mobility particle sizer has a condensation nucleus counter (CNC) designed to measure the number of particles while exponentially changing the voltage of the differential mobility analyzer over time. The number of particles thus measured is divided into particle numbers according to time intervals to thereby find particle concentrations relative to average electrical mobility in the respective time intervals. Particle size distributions are then found using a data on the particle concentrations.
However, the conventional scanning mobility particle sizer has a drawback in that it takes about two minutes to conduct the measurement once. This is because the scanning mobility particle sizer measures the particle size distribution based on the electrical mobility of the particles. Another shortcoming is that the scanning mobility particle sizer can be used only when the number of particles is as great as the number concentration in the atmosphere.